1. Field of the Invention
The present invention relates to a process for making a gas diffusion electrode for use in, e.g., an alkaline fuel cell and to a process for making the gas diffusion electrode.
2. Discussion of Background Information
An alkaline fuel cell (AFC) is an energy conversion device that converts the stored chemical energy of its fuel into electrical energy. Usually it uses hydrogen gas and oxygen gas to generate electrical power. The hydrogen is oxidized at the anode and oxygen (usually in the form of air) is reduced at the cathode. The product of the overall reaction is water.
AFCs were first employed by NASA for Apollo missions. Recently, Proton Exchange Membrane (PEM) fuel cells have found greater commercialization for a variety of reasons. However, the market is once again turning to AFCs because of several specific advantages. The advantages and disadvantages of AFCs are associated mainly with the use of an alkaline electrolyte, usually 7 M KOH or NaOH. With respect to advantages involved, circulating KOH helps to maintain the temperature and water balance and allows a simple design of the fuel cell. Further, the rate of electrochemical reactions in alkaline media is much higher than in acidic media (PEM), which makes it possible to use non-noble metal catalysts. Also, alkaline electrolyte permits broader operating temperatures than those of PEM fuel cells. AFCs can be operated over a wide temperature range, at temperatures as low as −10° C. and as high as +80° C. The construction of an AFC allows the liquid alkaline electrolyte to leave the cell after being shut down, and the overall lifetime of the electrodes is much higher than in the case of PEM fuel cells because corrosion occurs mainly when the cell is left at open circuit without load. Accordingly, the predicted service life of an alkaline fuel cell electrode is about 4000 hours. A disadvantage of an AFC is that the alkaline solution reacts with CO2 from air and forms carbonates, thereby plugging the pores of the electrode and decreasing the ionic conductivity of the electrolyte.
Fuel cells comprise a solid or liquid electrolyte and two electrodes, an anode and a cathode, at which the desired electrochemical reactions take place. Both electrodes are of the special porous type, called gas diffusion electrode (GDE). A GDE is in contact with reactant gas (e.g., hydrogen or oxygen (air)) at one side and with solid or liquid electrolyte at the other side. The electrolyte is in contact with both electrodes. The GDE allows the reactant gas to enter the electrode from the side of the electrode that is exposed to the gas supply, and to diffuse through the electrode to the reaction sites which contain catalyst for accelerating the electrochemical oxidation of hydrogen and reduction of oxygen. GDEs are usually double-layered, with a gas diffusion layer (GDL) that is to come into contact with reactant gas and an active layer (AL) that is to come into contact with the electrolyte. The GDL is designed to supply gas to the AL and at the same time to prevent the electrolyte from leaking through it.
There are two main mechanisms for gas transport in porous media, i.e., Fick diffusion and Knudsen diffusion. Fick diffusion (molecular or transport diffusion) occurs when the mean free path is relatively short compared to the pore size. It is applicable to Brownian motion, where the movement of each molecule is random and not dependent on its previous motion. Knudsen diffusion occurs when the mean free path is relatively long compared to the pore size, so the gas molecules collide frequently with the pore wall. Knudsen diffusion is dominant for pores having diameters between 2 and 50 nm and ensures a higher rate of gas supply to the AL than Fick diffusion.
The AL of a GDE is designed to optimize the contact between reactant gas, electrolyte and catalyst in a so-called three-phase boundary (solid-liquid-gas in the case of liquid electrolyte) to maximize the reaction rate. Catalysts are incorporated into AL structures to increase the rates of the desired reactions. Catalysts are often precious metals, particularly platinum or alloys thereof in a very high surface area form, dispersed and supported on high surface area electrically conducting porous carbon black or graphite (see, for example U.S. Pat. No. 4,447,505, the entire disclosure of which is incorporated by reference herein). The catalyst component may also comprise a non-precious metal, such as one of the transition metals. In fuel cells containing alkaline electrolytes the cathode may comprise catalysts based on, e.g., macrocyclic compounds of cobalt (see, for example U.S. Pat. No. 4,179,359, the entire disclosure of which is incorporated by reference herein). The ALs also comprise non-catalytic components in addition to the catalyst material, usually polymeric materials which act as binders to hold the layer together and may also have the additional function of adjusting the hydrophobic/hydrophilic balance of the final structure.
The hydrophobic binder, frequently polytetrafluoroethylene (PTFE), commercially known as Teflon®, is employed mainly in two forms, i.e., as a dry powder or as a suspension. One of the best ways is the preparation of a material called teflonized carbon black. This is done by mixing carbon black and a PTFE suspension. As a result a highly hydrophobic material with a high rate of gas diffusion is obtained. This material is described in, for example, U.S. Pat. Nos. 3,537,906 and 4,031,033, the entire disclosures of which are incorporated by reference herein, and has been used for the preparation of GDLs and as addition to ALs of GDEs.
Major problems with conventional GDEs relate to the prevention of leaking of a GDL and to keeping stable the three-phase boundary in an AL (see, e.g., U.S. Pat. Nos. 7,014,944 and 5,480,735, the entire disclosures of which are incorporated by reference herein). GDLs consist essentially of hydrophobized carbon materials such as carbon blacks. The leaking of electrolyte through the GDL is prevented by using hydrophobic material, such as PTFE, which also serves as binding agent. There is a relationship between the thickness and the hydrophobicity of a GDL—the lower the concentration of the hydrophobic material the thicker the GDL must be in order to prevent leaking A thicker GDL on the other hand, means a lower rate of gas supply to the AL. The stability of the three-phase boundary in the AL also depends on conflicting parameters: a higher concentration of hydrophobic binder will keep the pores dry to thereby ensure a sufficient supply of reactant gas, but it will also increase the thickness of the AL, thereby increasing the transport hindrances of reaction products in the liquid phase.
Thin layered GDEs are desirable also in order to provide a sufficient flexibility thereof. Lack of flexibility makes GDEs easily damaged on handling which leads to high reject rates during manufacturing of the electrode. This obviously has an impact on production costs.
In view of the foregoing facts one has to find the right balance of the electrode materials in order to achieve the best performance of the electrodes both from an electrical output and a mechanical stability point of view. Finding the best ratio of the materials for making electrodes is a major challenge in the development and production of any fuel cell, and of alkaline fuel cells in particular.